1. Field of the Invention
The present invention relates to a solid electrolytic capacitor element including a dielectric layer, a solid electrolyte layer, a carbon paste layer, and a conductive paste layer sequentially stacked on a surface of a valve acting metal substrate, which has a superior reflow resistance property, a manufacturing method therefor, and a solid electrolytic capacitor using the element.
2. Description of the Related Art
In recent years, electronic devices have been reduced in size, and adapted to digitization for saving electric power and higher frequencies, and the demand for solid electrolytic capacitors with low impedance at high frequencies, with high reliability, and with a high capacitance, has been increasing. As the capacitors having these performance qualities, capacitors have been commercialized which have a sintered tantalum body or aluminum foil as an anode and a highly conductive polymer solid electrolyte as a cathode. Also, in recent years, lead-free solder has been used to increase the temperature for surface mounting onto electronic circuit substrates. For this reason, there has been a need to employ a high heat-resistance material and a structure devised for relaxing thermal stress. The solid electrolyte cathode material has a low resistance, but exhibits poor performance when repairing the dielectric film. Thus, it is often the case that the dielectric film undergoes micro-destruction due to thermal stress at high temperature, thereby making it difficult to prevent an increase in the leakage current.
As examples of use of an insulating resin formed on a solid electrolytic capacitor element, Japanese Patent Application Laid-Open No. 07-094369 (Patent Document 1) discloses a solid electrolytic capacitor in which a solid electrolyte layer is formed on one surface of a valve acting metal divided by an insulator (masking) band, a separate insulator layer formed on a boundary section between the insulator (masking) band and the solid electrolyte layer, and a cathode leading conductor layer formed on the solid electrolyte layer so as to cover a portion of the separate insulator layer or so as not to come into contact with the separate insulator layer. This solid electrolytic capacitor is intended to prevent the material constituting the cathode leading from the conductor layer from penetrating into the discontinuity of the boundary between the solid electrolyte layer and the insulator (masking) band and come into direct contact with the anode oxide film to serve as a dielectric, thereby preventing an increase in leakage current. However, it is the initial characteristics before reflow that are improved by this method. In addition, this method forms an insulating resin on the solid electrolyte layer, and forms a graphite layer, thus reducing the current-carrying area covered with a highly conductive silver paint layer. Therefore, this method has a disadvantage in reducing the equivalent series resistance.
In addition, Japanese Patent Application Laid-Open No. 2007-294495 (Patent Document 2) discloses a stack-type solid electrolytic capacitor with a capacitor element which has an anode body including an anode section and a cathode section with a dielectric oxide film and a cathode layer sequentially formed on a surface of the anode body, in which an insulating resin layer is placed at and in the vicinity of the boundary between the cathode section and the anode section. This solid electrolytic capacitor construction is intended to relax the tension stress and bending stress at or in the vicinity of the boundary between the anode section and the cathode section during resistance welding of the anode section, due to the difference in thickness between the anode section and the cathode section. However, the insulating resin is also formed on a silver paint layer in this technique, thereby resulting in an increase in element thickness. This increase limits the number of elements which can be stacked in a standardized capacitor size.
Japanese Patent Application Laid-Open No. 2002-025863 (Patent Document 3) discloses a capacitor which has a low impedance and shows no increase in leakage current even when the capacitor is subjected to thermal stress or mechanical stress caused by reflow soldering or the like, by disposing a silver paste layer in an area of a carbon paste layer forming a cathode section and providing a gap between an end face of the silver paste layer and an end on a cathode side of an insulating layer serving as a boundary between an anode section and the cathode section. However, Patent Document 3 does not refer to the problem of the increase in temperature in the case of surface mounting to electronic circuit substrates through the use of lead-free solder.